Pluto

Glass positive of Pluto discovery, 1930

From 1930 until 2006, our Solar System had nine planets. Tiny Pluto was discovered by Clyde Tombaugh, who was searching for a predicted ‘Planet X’ that might explain oddities in the orbits of Neptune and Uranus. Tombaugh’s photographs show the same patch of sky a few nights apart. One ‘star’ seems to have moved, indicating that it is actually a planet. Astronomers later discounted the idea of Planet X – Pluto had just been in the right place at the right time. It was controversially demoted to ‘dwarf planet’ in 2006.
Pluto bumper stickers,

On 24 August 2006 the International Astronomical Union voted on a new definition of ‘planet’. The decision demoted Pluto to ‘dwarf planet’, reducing the number of planets in our Solar System to eight. This generated a flurry of worldwide news coverage about the changing status of our planetary neighbourhood. Internet ‘Save Pluto’ campaigns were quick to follow. These bumper stickers were among the first products to go on sale.